If you're only running a 32bit OS, then you're probably not making full use of that 4GB of RAM (you might be getting 3GB). Due to the large size of your rendering, you're probably running out of memory, even if your computer is telling you that you've got plenty of buffer space. You'll have to either upgrade to a 64bit OS with more memory (8GB should be plenty, but 4GB might work too), or render a smaller image (physically smaller, or at a lower resolution, your choice).
I replied to your AUGI post as well--
same answer as the other reply here; not enough RAM.
I wanted to close out this issue with a conclusion and give out a plug to those who were so helpful in responding and assisting with this issue.
The reason for the problem is just as Cliff and others surmised. My 32 bit machine has 4GB of memory, but only 2GB can be naturally allocated to one process. So, the guys at DCCADD, specifically Aaron, pointed me to a few hints to get an extra 1GB out of my machine. His suggestion is here:
Suggest customer enable 3GB switch per this document: http://usa.autodesk.com/adsk/servlet/ps/dl/item?si
All in all, not the result I wanted, but it's great to have a community and reseller that's on the ball. Kudos to everyone for their input.
I just had this problem. It would act like it was saving but the files would never appear. Very frustrating after a ten hour render. I took screenshots of it to make sure I had something. Then last ditch effort I closed the render dialog box and saved the project. Then before doing anything else, I opened the render dialog box again and clicked show render and tried to save/export again. And guess what? It worked!!! I have am not guaranteeing this will work every time or at all but It worked for me!